Boron filaments with a boron carbide antidiffusion coating,and metal matrix made therefrom

ABSTRACT

1. BORON FILAMENT HAVING A BORON CARBIDE COATING THEREON OF A COATING THICKNESS OF NO MORE THAN 15 MICRONS.

Nov. 5, 1974 v M. LECLERCQ ETAL 3,846,224 BORON FILAMENTS WITH A BORON CARBIDE ANTIDIFFUSION COATING, AND METAL MATRIX MADE THEREFROM Filed May 4. 1972 A 2 Sheets-Sheet 1 Nov. 5, 1974 LECLERCQ ETAL 3,846,224

BORON FILAMENTS WITH A BORON CARBIDE ANTIDIFFUSION COATING, AND METAL MATRIX MADE THEREFROM Filed May 4, 1972 2 SheetsSheet 2 FIG. 3

United States Patent France Filed May 4, 1972, Ser. No. 250,287 Claims priority, application France, May 11, 1971, 7116885 Int. Cl. C23c 1/08, 11/08 US. Cl. 161-170 6 Claims ABSTRACT OF THE DISCLOSURE Boron filaments having a boron carbide antidiffusion coating thereon of a coating thickness of no more than 15 microns. The boron filaments preferably have a tungsten boride core with a boron layer thereon. The coated filaments are useful for reinforcing metal matrices, particularly aluminium or aluminium alloy matrices. The coated filaments may be prepared by heating the filament to 900 to 1300" C. in the presence of hydrogen, a hydrocarbon, and a boron halide so as to deposit B 0 on the filament. An apparatus for production of the filaments comprises a tubular cell provided with gas inlet and outlet conduits and electrically-insulating plugs closing the ends of the cell. Each plug is provided with a passageway therethrough for the filament and is supplied with mercury from a supply conduit. The ends of each passageway are narrowed to substantially the diameter of the filament so that in use the mercury acts as an electrical contact with the filament to enable the latter to be heated by resistance heating yet is retained in the passageway by the narrowed ends to act as a gasket against gas flow through the passageway.

This invention is concerned with coated boron filaments and with a process and apparatus for the production thereof. I

It is known that boron filaments have mechanical properties which make them suitable for use as reinforcing agents in composite materials. For example, a boron filament 100,11. in diameter produced by heating a tungsten wire 12.5,; in diameter in the presence of a mixture of boron trichloride and hydrogen at the reduction temperature of the boron trichloride, has a mean strength of from 35 to 40 t./cm. and a modulus of elasticity of 4200 t./cm. for a density of 2.63 g./cm.

Composite materials consisting of a synthetic resin matrix reinforced with boron filaments are already in use in certain specialised industries (such as the aerospace industry), but their use remains limited because of the low mechanical strength of the resin,..particularly at high temperatures.

It would therefore be advantageous to use metal and metal alloy matrices in filament-reinforced composite materials, but the chemical reactivity of boron with metal matrices, for example aluminium, magnesium, nickel and titanium, makes it impossible to incorporate boron filaments directly in such matrices, irrespective of whether molten metal processing methods or metal spraying by means of plasma arcs are used.

For example, contact between aluminium and a boron filament at a temperature of about 650 0. results in degradation of the filament because of the formation of aluminium boride. The same applies to spraying of this metal by a plasma arc torch. .A boron filament borided in this way loses 80% of its tenstile strength.

We have now discovered that boron carbide (B C) coatings provided on boron filaments reduce the diffusion between the boron and the metal matrix yet do not substantially affect the mechanical properties of the filament.

According to the present invention we therefore provide a boron filament having a boron carbide coating thereon of a coating thickness of no more than 15 microns, and preferably from 3 to 8 microns.

We also provide a method of producing a boron filament having a boron carbide coating thereon, which comprises heating a boron filament at a temperature of from 900 to 1300 C. (preferably 1150 to 1300 C.) in the presence of a gaseous mixture of hydrogen, a hydrocarbon (such as methane), and a boron halide (such as boron trichloride), the proportions of the gases being sufiicient to form B C, for a sufficient length of time to provide a boron carbide coating on the filament of no more than 15 microns in thickness. The contact time of the filament with the gaseous mixture is preferably from 4 to 6 seconds. If desired, the proportion of the hydrocarbon and/or hydrogen in the gas mixture may be in excess of that stoichiometrically required to form B 0.

In a third aspect of the invention, we provide an apparatus for providing a boron filament with a boron carbide coating thereon which comprises a tubular cell provided with inlet and outlet conduits for the supply and removal of gas, the cell being closed at each end with a plug of electrically-insulating material, each plug having a passageway for the boron filament to pass therethrough, each passageway communicating with a conduit for the supply of mercury thereto and the ends of each passageway being narrowed such that, in use, mercury supplied to the passageway provides an electrical contact with the boron filament passing therethrough but is retained therein by the narrowed ends so as to act as a gasket against flow of gas through the passageway.

The boron carbide coating creates an effective antidiffusion barrier and enables practically all the mechanical properties of the initial boron filament to be retainedafterv aluminium has been sprayed thereon by a plasmaarc torch, so as to give boron filament-reinforced aluminium matrix composite material. A study of X-ray diffraction spectra has shown that there is nointeraction between the coated boron filaments and the aluminium matrix...

The antidiifusion effect of the boron filaments accord;- ing to the invention may be evaluated as follows:

.After plasma arc torch spraying with aluminiumto give a boron filament-reinforced aluminium matrix composite, the aluminium matrix is dissolved by meansof concene trated caustic soda solution and the boron carbide coated. boron filaments are recovered. The mean value ofthe tensile strength of the filaments is then determined. Let R be the mean value of the tensile strength of the coated. boron filament prior to formation of the composite and let R be this mean value after the coatedfilamentshave been sprayed with aluminium and then recovered. The antidiffusion effect maybe defined by the ratio R/R .(and is equal to a maximum of 1).].

The following results were obtained with a boron filaments initially 98 microns in diameter (the filamentnhad,

a 15 micron diameter tungsten boride. core):.-:

7 Thickness of Antidif- I i the B 0 coating R fusion (microns) (t/cmfi) (t/em effect r.

*This thickness of the boron carbide coating is defined as the ditference between the diameter of the boron carbidecoated boron filament and the diameter of the bare, initial boron filament.

The ratio R/R is approximately equal to 1 when the thickness of the B coating is from 3 to 8 microns; it is slightly less than 1 for other thickness values of less than or equal to microns. The tensile strength of the coated filaments according to the invention is greater than that of the initial uncoated filaments when the thickness of the B C coating is from 3 to 8 microns, and is comparable to that of the initial uncoated filaments for other thickness values of less than or equal to 15 microns. Thus, with a boron filament initially 98 microns in diameter and having a 15 micron diameter tungsten boride core, the following results for the tensile strength were obtained:

1 Initial boron filament.

The tensile strength of the coated filaments drops considerably, however, when the B 6 coating is greater than 15 microns. It is surprising that the boron carbide deposit on the filament does not substantially reduce the tensile strength of the initial boron filament in the above-mentioned thickness range, and that in some cases it even improves the tensile strength.

-In the method of the invention any excess of hydrocarbon in the gaseous mixture may be as much as or more (compared to the stoichiometric amount), and, if desired, any excess of hydrogen may be as much as 100% compared to the stoichiometric amount. Preferably, the boron filament is heated to the required temperature by passing an electric current through the filament and causing resistance heating. The filament is typically passed through a chamber which is continuously supplied with the gaseous mixture, the reaction taking place in this chamber.

The apparatus of the invention is preferably located downstream of the reaction chamber of a boron filament production machine wherein a tungsten wire is heated in the presence of a mixture of boron halide and hydrogen as described in French Patent Application No. 6,901,287 to form a boron coating on the tungsten wire.

In one embodiment of the apparatus of the invention, the reaction cell comprises, in addition to gas inlet and outlet conduits, disposed at, or adjacent, the plugs, an inlet conduit for hydrocarbon disposed intermediate the ends of the cell. This apparatus is particularly suited to depositing boron on a tungsten wire in the same chamber as the deposition of the boron carbide: a mixture of boron halide and hydrogen is introduced to the cell via the gas inlet conduit at the upstream end of the cell (to produce a zone for deposition of boron on to, for example, a tungsten wire entering the cell) and the hydrocarbon (e.g. methane) is introduced through the additional conduit (to complete the gas mixture so as to provide a zone for deposition of boron carbide downstream of the boron deposition zone).

The production of a boron carbide-coated filament according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a cross-section through an apparatus according to the invention for the deposition of a boron carbide antiditfusion' coating on to a boron filament;

FIG. 2 is a modification of the apparatus shown in FIG. 1 and provides for the deposition of boron and then boron carbide in one reaction chamber; and

FIG. 3 is a diagram of an installation for the continuous production of boron filaments and then the coating thereof with boron carbide.

The apparatus shown in FIG. 1 comprises a cell consisting of a Pyrex cylindrical glass tube 1 closed at the ends by Teflon plugs 2 having flanges 2a over which the ends of the tube are force-fitted with the interposition of ring gaskets 3. Each plug is formed with a longitudinal passage 4 for the boron filament being coated and is pro- 7 vided with glass half-capillaries 5, the free ends of which are narrowed to leave an aperture corresponding roughly to the diameter of the filament passing therethrough.

A radial conduit 6 is provided at the lower part of each plug, communicates with the passage 4, and is intended for the supply of mercury 7 to the passage 4 from a reservoir (not shown). The mercury supply is connected to an electrical current source and acts both as a gasket. for the cell and as an electrical contact for the filament tional tensioning device which forms part of the boron filament production machine.

Typical operating conditions were as follows: using a cell 30 cm. in length, the boron filament 10 was heated by the Joule elfect (electrical resistance heating) to a.

temperature of 900-1300 C. and was driven at a speed of from 60 to 400 metres per hour using a mechanical tension of 10-100 g. A mixture of boron trichloride,

methane and hydrogen in stoichiometric proportions in accordance with the following reaction equation was introduced into the cell via the conduit 8:

4 BCl +CH +4H B C+ l2HCl Boron filaments with a boron carbide antidiffusion coating were produced, the mechanical characteristics of which were substantially the same as those of the initial boron filament. The results of these examples are given in the Table I hereinafter. In these examples, the rate of flow of hydrogen was the same as that of BCl while the rate of flow of methane was 25 of that of BCl From Table I it may be seen that the optimum conditions are obtained with contact times ranging from 4 to 6 seconds, in which case the temperature can range from 1150 to 1200 C. approximately (with the gas quantities indicated) depending upon the required deposition thickness. In the case of coatings thicker than 8 microns, the breaking load starts to fall, the tensile strength (load with respect to section) being highest in the case of 3-8 micron thick coatings.

When an excess of hydrogen is used with respect to the stoichiometric quantity (for example when doubled), comparable results are obtained. However, when an excess of methane is used with respect to the stoichiometric quantity, the breaking load of the filament is increased as is also its tensile strength and this appears to be the result of the particular structure of the deposit.

The results of further examples are shown in Table II hereinafter. In these examples, the rate of flow of hydrogen was the same as that of BCl while the rate of How of methane was 35% of that of BCl The mean value of the tensile strength of the resulting boron carbide-coated boron filaments was from 35 t./cm. to 40 t./cm.

In the modification of the apparatus shown in FIG. 2, the tube 1 of the coating cell is additionally provided with a conduit 12 for the admission of methane (or other hydrocarbon such as ethane, benzene, or toluene), this conduit 12 is situated at a suitable distance from the rear end of the cell, for example cm. therefrom. The mixture of boron trichloride (or other boron halide, such as boron tribromide) and hydrogen is admitted to the cell via con duit 9 and the gases are discharged via conduit 8 after reaction in the cell.

The portion of the cell situated upstream of conduit 12 forms a boron deposition Zone while the portion downstream of conduit 12 acts as a boron carbide deposition zone.

FIG. 3 shows a complete schematic installation for the production of boron filaments coated with a boron carbide antidiffusion coating from a tungsten wire. This installation comprises the following, in a manner known per se:

Rate of flow of gas in cell C:

methane (preheated to 250 The resulting coated filament had a total diameter of 105 microns, a 5 micron thick boron carbide deposit, and a tensile strength of t./cm.

TABLE I Rate of flow Diameter of Breaking load Diameter of Breaking load Reaction of B013 initial boron of irutlal Temperature B coating of boron cartime (cm per filament boron filafilament to filament bide coated (seconds) minute) (microns) ment (kg.) 0.) (microns) filament (kg.)

TABLE II Rate of flow Diameter of Breaking load Diameter of Breaking load Reaction of B01 initial boron of imtial Temperature B 0 coating of boron eartime (cm per filament boron filaof filament to filament bide coated (seconds) minute) (microns) ment (kg.) 0.) (microns) filament (kg.)

a cell A for cleaning and degassing the tungsten wire to be We claim:

coated; a cell B for the deposition of a first boron coating on the wire; a cell C (the apparatus according to the invention as described above with reference to FIG. 2, for the successive deposition of a second boron coating and a boron carbide antidifiusion coating. Cell B which is optional, is connected to cell C by means of a connection 2' (see FIG. 2), produced by modifying plug 2 by adding a reaction gas discharge duct 8 and flanges Z'a.

The cells are fed with boron trifiuoride (or other boron halide) through circuit D, with hydrogen through circuit E, and with methane (or other hydrocarbon) through circuit G, via the calibrated flow meters F. After reaction, the gases are discharged through circuit H.

The tungsten wire is taken from the feed spool 13 through the different cells by the take-up spool 15 which is driven by a variable-speed motor. The mechanical ten sion of this wire is controlled by a tensioning device 14 situated immediately after the feed spool.

Typical operating conditions for this installation were as follows:

Rate of flow of gas in cell B:

hydrogen "litre per minute 1.8 boron trichloride do 1 Contact time in cell B seconds 20 Maximum temperature of filament in cell C C 1300 1. A boron filament having a boron carbide coating thereon of a coating thickness of no more than 15 microns.

2. A filament as claimed in claim 1 wherein said coating thickness is from 3 to 8 microns.

3. A filament as claimed in claim 1 wherein said boron filament consists of a tungsten boride core provided with a boron coating thereon.

4. A filament as claimed in claim 3 wherein said boron filament has a diameter of at least 98 microns.

5. A composite material which comprises a metal matrix reinforced with one or more of said boron filaments as claimed in claim 1.

6. A composite material as claimed in claim 5 wherein said metal matrix is selected from the group consisting of aluminium or an aluminium alloy.

References Cited UNITED STATES PATENTS 3,556,836 1/1971 Basche et a1 117-107.1 3,668,006 6/ 1972 Higgin et al. 117-106 3,574,649 4/1971 Fanti et al 117-106 3,565,683 2/ 1971 Morelock 117-215 3,700,486 10/1972 Veltri et a1 117-71 3,410,715 11/1968 Hough 117-231 3,409,469 11/ 1968 Kuntz 117-231 3,491,055 1/ 1970 Talley 161-170 RALPH S. KENDALL, Primary Examiner I. W. MASSIE, Assistant Examiner US. Cl. X.R. 

1. BORON FILAMENT HAVING A BORON CARBIDE COATING THEREON OF A COATING THICKNESS OF NO MORE THAN 15 MICRONS. 